1. Field of the Invention
The present invention relates to camera in which the image of a photographic subject is converted into digital data, and the data is compressed and stored on a recording medium. More particularly, the present invention relates to a still camera which automatically selects a data compression factor for data compression processing.
2. Description of the Related Art
Still cameras are know which record the image of a subject in memory as digital image data. For example, FIG. 3 is a block diagram illustrating an example of a prior art imaging system for a still camera which records the image of a subject in memory as digital data. According to the system shown in FIG. 3, an imaging action is begun in response to the half-depression of a release button (not shown). Light from a photographic subject enters a lens 50 and is imaged on an imaging unit 54. The amount of light imaged on the imaging unit 54 is controlled by an electronic shutter (not shown) and a stop 52. The imaging unit 54 photoelectrically converts the light from the photographic subject into analog electrical signals by a built-in charge coupled device (CCD), or similar device. A signal processing unit 56 receives the photoelectrically converted output of the imaging unit 54 and performs white balance correction, amplification, .gamma. correction and like processing on the received signals to produce an image signal. The image signal output from the signal processing unit 56 is converted into a digital image signal by an analog-to-digital (A/D) converter 58, and the output of the A/D converter 58 is temporarily stored in a buffer memory 60. The image signal data stored in the buffer memory 60 is sent to a data compression unit 62 when the release button is fully depressed. The data compression unit 62 performs well-known data compression processing on the image data, and, finally, the compressed image data is stored in memory on a memory card 64. A control unit 66 is also provided which controls each of the above-described circuits.
The prior art imaging system shown in FIG. 3 may operate in two modes: a manual compression mode, and an auto compression mode. The mode of data compression by the data compression unit 62 can be selected by the photographer, and data compression is performed according to the selected data compression mode. When operating in the manual compression mode the photographer may select a data compression factor. However, when performing imaging in the auto compression mode, the action of the data compression unit 62 is carried out according to the properties of the digital image data stored in the buffer memory 60, or by extracting a portion of image data, and from the data length at this time, the camera selects an appropriate compression factor, and data compression is performed according to the selected compression factor.
A system for automatically selecting a compression factor is known. For example, Japanese Laid-Open Patent Publication 2-257780 discloses various methods for performing data compression of digital image data. According to one method disclosed, data compression is performed at a suitable data compression ratio such that the smallest amount of data remains after compression. According to another method disclosed, the compression factor is selected such that a fixed amount of data remains after compression.
However, in the prior art imaging system for a video still camera described above, when a photographer has selected the auto compression mode, compression factor selection is performed by actually performing data compression processing for the digital image data until arriving at the selection of a compression factor. The data compression process, therefore, has to be performed a number of times to select a compression factor, and, as a result, the process is lengthy. Therefore, the time interval from the commencement of the photographic action until the storage on the memory card 64 is long. This time interval is a primary factor preventing the speeding up of continuous photography. Furthermore, a large amount of digital data is necessary for appropriate compression factor selection. The amount of data required for appropriate compression lengthens the time necessary for selecting a compression factor. Further, since, according to the prior art, the data compression unit 62 performs data compression until a compression factor is selected, electrical power is wasted.